Well, Morgana's reaction could be quite nuanced. She may feel betrayed that Merlin has kept this secret from her all this time, especially if she thought they were close friends. However, she might also quickly realize the power they could have together. Her reaction could also depend on the context within the fanfiction. For example, if Camelot is in a time of great danger, she may be more accepting of Merlin's magic as a means to save the kingdom.
She might be shocked at first. Given that in the original story Morgana has her own complex relationship with magic, seeing Merlin's magic could stir a mix of emotions in her, like envy or a sense of relief that she's not alone in her magical abilities.
Morgana could initially be frightened. Merlin has been hiding his magic for so long, and suddenly revealing it is a big deal. She might fear what this means for her, for Camelot, and for their relationship. But as she processes it, she may also start to see the potential of them working together with their magic.
She could be shocked at first. After all, Merlin has been hiding his magic for so long. Maybe she feels a sense of betrayal initially, but then she might see an opportunity to have an ally in the world of magic.
Uther might initially be shocked. He has always been so against magic, so he could be furious at first. He might feel betrayed by Merlin, whom he has known for a while.
A fanfiction could focus on the relationship between Mordred and Morgana. It could explore how Mordred came to be so loyal to Morgana in the first place. Was it out of fear, admiration, or something else? And how does Merlin fit into this complex dynamic? He might be seen as an outsider trying to break their bond.
In some fanfictions, Prince Merlin (assuming he has some sort of princely status in this AU) and Morgana might be forced into an alliance against a common enemy. Maybe an ancient evil that threatens Camelot in a way that neither of them can face alone. They would have to put aside their differences and work together, which could lead to a slow - burning romance.
He could face Morgana's distrust if she takes his secrecy the wrong way. Maybe she'll think he has other motives for hiding his magic from her.
Their relationship could develop in a complex way. If Morgana ties up Merlin out of anger or suspicion initially, but then realizes she was wrong, it could lead to her feeling guilty. This might make her more open to Merlin's friendship in the future, and they could gradually build trust.
The storyline might progress with a lot of drama. People's reactions would be the key factor. If he reveals it to Arthur, for instance, Arthur's response could either make their friendship stronger or break it apart. This would then lead to different paths for Merlin, either having to hide again or standing up against all odds.
In fanfiction, Evil Merlin can be depicted as a mastermind behind many dark plots, while Morgana is the face of the chaos they create. For instance, Merlin might be the one secretly pulling the strings, using his magic to make things happen, and Morgana is the one out in the open, using her charm and power to intimidate others. They are like two sides of the same evil coin, complementing each other in their wrongdoings.
One possibility is that there would be a great deal of tension. Old Merlin has seen so much and has his own wisdom, while Morgana has her own powers and motives. Maybe Merlin would try to persuade Morgana to abandon her dark path. He could use his experience and the bond they once had to appeal to her better nature.
It often starts with a spark of curiosity or an unexpected moment of connection. Maybe they see a different side of each other during a magical encounter. Then, as they interact more, they start to understand each other on a deeper level. Their relationship might develop slowly, with small acts of kindness and shared secrets.